While Battlestar Galactica is seen as the sci-fi show that is most clearly a thematic reaction to 9/11 and the 'post 9/11 world' (complaints about how its ripped from the headlines nature negate it's own ability to world-build notwithstanding), something I've started to noticed is sci-fi shows using the imagery of 9/11 in a distinctive way quite separated from delving into the events themselves. One of most famous reveals in cinematic sci-fi is from The Planet of the Apes, where Charton Heston finds the Statue of Liberty buried in the beach and realises this place that he thought was an alien wilderness is in fact all too familiar. What’s cropping up in a few places now is the World Trade Center working as the reverse of that, a reveal of a New York landmark that shows you have left the familiar and entered a strange, unsettling place.
In both Fringe and the US-remake of Life on Mars, the presence of the World Trade Center serves this function, solidifying that characters have entered a strange new world. In Life on Mars, Sam Tyler seeing the intact WTC is the crown of the accident sequence, finishing his transition to the alien world of yesterday.
A major feature of Fringe is the presence of an alternate universe that is fundamentally both similar and different to our own. At the end of the first season of Fringe, Olivia has travelled to meet the enigmatic William Bell only to whisked away to somewhere else - the final shot of the series revealing she has travelled to the other universe and to an office in a still-standing alternate version of the World Trade Center. While the difference in the events of 9/11 are far from the only feature separating the universes (there’s something interesting about growing numbers of alternate universes with zeppelins but anyway) it’s used as a continuing touchstone for stories involving the other universe; the repeated use of the NY skyline, an architect from ‘Other There’ caught in a fringe event was designing the New Pentagon, Peter’s visit to a possible future has him arriving at the base of a completed Freedom Tower, the ‘amber-blue’ Lincoln Lee on crossing over for the first time doesn't think the other universe seems that different until he gets his first look at the twin towers - showing him even though the place is superficially familiar it is deeply alien.
With their shape they invoke another icon of classic sci-fi: the monolith. When I first thought about this I dismissed it as a bit of a trite comparison between two things that happen to be cuboids - but the way they're used is actually quite similar; they're deployed as a symbol of strangeness beyond understanding, something that couldn't possibly exist and yet does.
That the past is many ways a different planet is part of the fundamental conceit of Life on Mars and for all the discussion of whether 9/11 really did "change everything", it's interesting to see how quickly it's entered the sci-fi repertoire as part of how we describe an utterly alien world.